For our son we transitioned him cold turkey from his crib to his twin bed at age 2. We needed the crib for his baby sister who was to arrive a few months later.
By 16 months he was already climbing out of his crib anyways.
For us getting a toddler bed seemed like a waste of money since in a couple of years we would have to buy a twin bed for him anyways.
So we bought him a brand new mattress and box spring. Then to save us a bunch of money I found a really nice, but simple wood bedframe at a thrift store. It cost me $50 plus $5 for some additional screws at Home Depot.
We also bought the First Years Double Sided mesh bed rail from Target. It was pretty easy to set up. We only used it for a few months with my son as he quickly got used to his Big Boy Bed and didn't fall out of bed at all. In fact, after two months of using it, he figured out on his own how to pull it out from under his mattress and one morning we found him sleeping soundly in the middle of his bed with the bed rail in the middle of his bedroom on the floor. So we removed it permanently from that point on. Now we are saving both sides for our DD who will probably get her Big Girl Bed in another year or so.
Initially with my son I think my hubby slept in the new bed with him for a couple of nights. After that we had no problems. We just put up one of those cheap wood gates from Wal Mart on my sons door to keep him from roaming the house if he got up too early. Now it seems that having that wood gate up on his door gives him a sense of security, even though he can totally climb over or remove it himself to go potty or to come and get us in the morning.
Another thing that I did to get my son excited about sleeping in his Big Boy Bed was to take him to Target and Wal Mart so he could pick out his own sheet and comforter set. He picked out a cute dinosaur set that he still loves today. In fact he won't go to sleep unless he has his "Dino blankie". He doesn't care what sheets are on the bed, but he has to have his Dino blankie/comforter.